1. Field of the Invention
The present invention relates to an internal combustion engine and a motorcycle equipped with the internal combustion engine.
2. Description of the Related Art
Conventionally, in an internal combustion engine for motorcycles and the like, oil is circulated in oil passages formed in various parts, such as a cylinder head, a cylinder body, and a crankcase. The oil having been circulated through the various parts of the internal combustion engine is returned to an oil pan provided below the crankcase and is again sent out to the various parts. In this way, oil is circulated in the internal combustion engine.
In one example of the oil circulation, the oil circulated in the cylinder head flows into a cam chain chamber provided at a side portion of the cylinder body, and returns to the oil pan through a crank chamber for accommodating a crankshaft. JP 2011-38437 A discloses an internal combustion engine having a communicating groove allowing communication between a hermetically closed crank chamber and a cam chain chamber.
In the crank chamber, the reciprocating motion of a piston causes a crankshaft to rotate at a high speed. Therefore, pressure variations occur in the crank chamber. The pressure variations hinder the oil circulating in the cam chain chamber from flowing into the crank chamber. This may cause the problem of degradation in the oil circulation performance. In view of such a problem, it may appear possible to perform the oil circulation efficiently by additionally providing a bypass passage for returning the oil directly from the cam chain chamber to the oil pan. However, additionally providing the bypass passage may result in a cost increase. Moreover, since there are constraints on the component layout in an internal combustion engine, the provision of the bypass passage may result in a size increase of the internal combustion engine.